SOME  PRINCIPLES 

OF 

SAFETY  VALVE  DESIGN 


WITH  SPECIAL  CONSIDERATION  OF  VALVE  SPRINGS 
FOR  LOCOMOTIVE  POP  SAFETY  VALVES 


BY 

ALFRED  B.  CARHART,  E.  E. 

MEM.  AM.  SOC.  M.  E. 


Reprinted  from  papers  presented  before  The  American  Society  of 
Mechanical  Engineers 


CROSBY  STEAM  GAGE  &  VALVE  COMPANY 


F.  887-B.  P.-4-09 


19  0  9 


C\3so 


Safety  Valve  Springs* 

THE  spring  is  the  heart  of  the  modern  pop  safety  valve;  it  is 
the  essential  element,  to  which  all  other  details  of  design  are 
subordinate.  If  the  spring  is  not  properly  proportioned  for  its 
purpose,  the  satisfactory  performance,  discharge  capacity  and  durability 
of  the  valve  can  not  be  secured.  The  operations  of  a  poorly  designed 
valve  may  be  greatly  assisted  and  improved  by  a  suitable  spring;  and  a 
valve  with  an  excellent  arrangement  and  relation  of  the  other  working 
parts  and  discharge  passages  may  be  seriously  handicapped  by  an 
improperly  designed  spring  or  even  transformed  from  a  safety  device  to 
a  source  of  danger. 

The  history  of  safety  valve  making  indicates  that  the  spring  has 
been  regarded  as  a  relatively  unimportant  detail,  made  up  without 
exact  knowledge  or  study,  following  any  general  custom  or  usage  that 
had  seemed  satisfactory  in  the  past,  and  doing  as  well  as  might  be 
within  the  dimensions  conveniently  available  in  valves  designed  prima¬ 
rily  with  regard  to  economy  of  material,  pleasing  proportions  and  uni¬ 
form  gradations  of  size,  thus  forcing  all  sizes  of  springs  for  wide  ranges 
of  pressure  to  go  into  the  same  body  or  casing. 

A  suitable  and  efficient  spring  must  be  the  first  consideration.  A 
safety  valve  should  be  designed  by  calculating  the  total  spring-load 
required  to  be  exerted  upon  the  disc  when  the  valve  is  closed,  then 
the  amount  of  further  compression  needed  for  the  pre-determined 
vertical  lift  of  the  disc  when  the  valve  opens,  with  a  reasonable  allowance 
for  a  reserve  of  further  possible  free  movement  of  the  spring  in  com¬ 
pression,  and  thereupon  determining  the  dimensions  of  the  spring  that 
will  carry  this  load  at  its  point  of  greatest  efficiency,  with  due  regard 
for  flexibility,  sensitiveness  with  accurate  adjustment,  and  durability 
in  service. 

There  are  several  variable  dimensions  and  factors  that  may  be  to 
some  degree  modified,  in  proper  relation  to  each  other,  in  designing 
springs  to  meet  any  given  requirements,  and  several  different  springs 
may  be  laid  out  that  will  accomplish  much  the  same  result^  with  but 
slight  differences  in  allowance  for  the  several  factors  that  must  be 
assumed;  but  there  is  a  certain  minimum  limit  beyond  which  it  is 
impossible  to  go,  and  where  the  available  space  has  been  by  custom  or 
commercial  considerations  absolutely  fixed,  the  obvious  procedure  is 

♦Presented  at  the  Meeting  of  The  American  Society  of  Mechanical  Engineers 
held  in  New  York  on  February  23,  1909. 


4 


SAFETY  VALVE  SPRINGS 


not  to  let  it  all  go  as  a  difficult  problem  having  no  rational  solution,  but 
to  make  the  spring  of  a  steel  that  permits  greater  fibre  stress,  to  design 
an  arrangement  of  valve  parts  which  shall  have  the  least  throttling  or 
retarding  effect  upon  the  steam  which  the  spring  does  permit  to  escape, 
and  to  utilize  all  means  to  reduce  the  required  total  spring-load  to  the 
least  amount  without  restricting  the  valve  outlet  diameter  or  area. 
Details  of  mechanical  construction  are  thus  important,  but  secondary, 
for  the  efficiency  of  the  spring  in  its  functions  will  after  all  determine 
the  satisfactory  performance  of  the  valve.  Modifications  in  valve 
design  may  help  but  can  not  cure  a  viciously  ineffective  spring.  On 
the  other  hand,  it  is  not  always  possible,  within  the  limits  imposed,  to 
design  a  spring  to  overcome  or  counteract  defects  of  valve  design. 

Some  of  the  characteristics  of  helical  springs  are  commonly  known, 
but  it  may  be  well  to  review  briefly  the  principles  involved.  The 
accepted  foundation  of  the  mathmetical  theory  of  elastic  solids  is 
Hooke’s  Law,  “Ut  tensio  sic  vis,”  or,  as  the  strain  is  So  is  the  stress,  and 
this  is  true  of  the  spring,  that  within  the  limits  of  elasticity,  the  de¬ 
formation  or  compression  is  proportional  to  the  force  or  pressure  which 
produces  it;  and  in  a  spring  of  given  dimensions,  equal  increments  of 
force  or  pressure  applied  will  produce  equal  amounts  of  compression. 
For  example,  if  it  requires  a  total  load  of  2000  pounds  to  compress  a 
given  spring  having  a  total  possible  compression  of  one  inch  so  that  its 
coils  are  solid,  with  no  further  deflection  possible,  then  a  load  of  1000 
pounds  would  have  caused  this  spring  to  shorten  just  one  half  of  that 
amount  or  one-half  inch,  and  each  100  pounds  load  more  or  less  would 
cause  a  shortening  or  lengthening  of  1-20  or  .05  inch.  The  total  amount 
of  compression  or  shortening  of  a  spring  under  load  is  divided  equally 
among  its  free  or  effective  coils,  and  as  the  amount  of  deflection  under 
a  given  load  is  determinable  by  formula  or  by  experiment  for  every 
combination  of  size  of  steel,  mandrel  diameter  and  pitch,  the  total 
compression  of  a  spring  is  found  by  multiplying  the  deflection  of  each  coil 
by  the  number  of  effective  coils;  and  for  any  fixed  proportions  of 
diameter  and  pitch,  the  compression  of  a  spring  at  any  given  load  is 
proportional  to  the  number  of  coils;  therefore  it  is  generally  said  that 
the  long  springs  have  greater  compression  than  short  springs  and  that 
the  simplest  way  to  increase  the  total  compression  or  movement  is  to 
lengthen  the  spring.  But  of  course  an  increased  compression  for  a 
given  load  can  be  accomplished  by  changing  other  proportions  without 
increasing  the  total  free  length. 

This  increase  of  compression  of  a  spring  in  proportion  to  the 


SAFETY  YALYE  SPRINGS 


5 


increasing  number  of  its  coils  is  independent  of  the  total  load  which 
the  spring  will  carry  and  does  not  affect  that  question.  The  strength 
of  a  spring,  or  the  load  which  it  will  carry  or  sustain  at  any  given  pro¬ 
portionate  amount  of  its  total  possible  compression,  is  determined  by 
the  characteristics  of  the  metal,  the  size  or  cross-section  of  the  rod  or 
wire  and  the  diameter  of  mandrel  on  which  it  is  wound;  for  example, 
if  a  load  of  1000  pounds  will  compress  a  spring  of  certain  diameter- 
dimensions  one-half  of  its  total  possible  compression  or  one-half  inch 
then  a  spring  of  the  same  diameters  but  twice  as  long  and  having  double 
the  number  of  coils  would  be  compressed  by  the  same  load  one-half  of 
its  total  movement  or  one  inch;  a  load  of  1500  pounds  would  compress 
either  spring  three-quarters  of  its  total  possible  movement,  and  likewise 
either  spring  would  be  compressed  solid  under  a  load  of  2000  pounds; 
the  strength  or  carrying  capacity  of  the  two  springs  would  be  the  same, 
although  of  different  lengths,  and  the  amount  of  deflection  of  each  coil 
in  either  spring  would  be  the  same;  although  the  total  amount  of  move¬ 
ment  would  be  different,  a  spring  four  inches  long  would  be  just  as  strong 
or  would  sustain  the  same  load  as  a  spring  eight  inches  long;  there  would 
be  no  difference  in  their  capacity  or  load  strength.  But  the  action  of 
the  two  springs  in  safety  valve  service  would  be  very  different;  for 
the  longer  spring  would  have  its  power  exerted  through  a  greater 
distance,  with  greater  amplitude  of  movement  or  flexibility,  and  the 
oscillations  set  up  by  any  sudden  force  would  affect  the  behavior  of 
the  valve. 

If  all  other  dimensions  remain  unchanged,  enlarging  the  cross 
sectional  area  of  the  rod  or  wire  of  which  the  spring  is  wound  will  make 
the  spring  stiffer  or  stronger  or  carry  a  heavier  load  for  the  same  amount 
of  compression;  and  of  course  square  rod  will  make  a  spring  slightly 
stiffer  than  round  steel  of  the  same  diameter.  There  are  some  condi¬ 
tions  which  may  be  better  met  and  special  results  that  can  be  more 
readily  obtained  by  using  round  section  rather  than  square,  and  vice 
versa;  and  generally  the  controlling  consideration  is  not  merely  the 
greater  area  or  volume  of  the  steel,  but  rather  a  slightly  different 
characteristic  reaction  and  activity,  which  can  be  considerably  modified 
by  slight  alteration  of  other  elements  of  the  design.  Reducing  the 
mandrel  diameter  will  also  make  the  spring  stiffer  or  stronger  and  thus 
admit  less  compression  for  the  same  load;  while  increasing  the  inside 
diameter  or  mandrel  upon  which  the  spring  is  wound  will  make  the 
spring  weaker  or  more  flexible,  and  will  decrease  the  amount  of  the 
load  which  it  will  sustain  or  carry  for  a  given  compression  or  will  increase 


6 


SAFETY  VALVE  SPRINGS 


the  deflection  of  each  coil  and  likewise  the  total  compression  which 
carries  or  balances  a  given  pressure  load. 

Therefore  the  total  amount  of  compression  of  a  spring  for  a  given 
load  may  be  increased  by  increasing  the  number  of  coils  of  the  same 
diameters  and  pitch  and  thus  increasing  the  total  free  length;  by  reduc¬ 
ing  the  cross-section  area  of  the  rod;  by  enlarging  the  mandrel  and 
consequently  also  the  over-all  diameter;  or  by  any  or  all  of  these  modifi¬ 
cations  at  the  same  time. 

In  making  a  safety  valve  spring,  there  are  certain  practical  limita¬ 
tions  which  must  be  taken  into  account.  Steel  of  very  large  diameter 
cannot  be  satisfactorily  wound  upon  a  small  mandrel,  and  the  flexural 
or  bending  strain  becomes  too  great,  so  that  a  fracture  or  transverse 
set  is  developed.  A  spring  excessively  long  in  proportion  to  its  diameter 
and  pitch  may  bend  or  buckle  instead  of  compressing  in  a  straight 
thrust  axially;  and  if  the  number  of  coils  be  too  great,  the  reaction  of 
the  spring  will  set  up  an  oscillation  which  not  only  permits  but  ag¬ 
gravates  the  undesirable  and  destructive  chattering  of  the  valve. 
The  valve  disc  must  not,  in  effect,  be  suspended  at  the  end  of  a  flexible 
spring,  but  must  have  behind  it  at  all  times  a  positive  force  capable  of 
controlling  its  action  when  lifted  by  the  escaping  steam.  If  the  spring 
be  too  short,  not  only  will  the  reaction  be  too  sudden,  but  the  active 
free  coils  will  form  a  smaller  proportion  of  the  total  length;  it  is  not  then 
possible  to  distribute  the  pressure  at  the  ends  of  the  spring  so  evenly 
upon  the  coils,  and  the  spring  compression  will  be  greater  on  one  side 
than  on  the  other,  transmitting  an  undesirable  side  thrust  to  the  disc 
guides.  If  the  pitch  is  too  steep  or  the  coils  wound  too  far  apart,  there 
will  be  room  for  considerable  free  movement  and  apparently  a  large 
possible  deflection  of  each  coil,  so  that  it  might  seem  as  though  only  a 
few  coils  would  give  the  total  compression  desired;  but  if  this  be  at¬ 
tempted,  the  fibre  stress  upon  the  steel  is  enormously  increased,  the 
flexural  strain  becomes  the  larger  factor,  the  rod  is  fractured  or  the 
elastic  limit  of  the  steel  is  soon  passed  and  a  permanent  set  takes  place, 
destroying  the  essential  properties  of  the  spring  before  the  required 
deflection  of  each  coil  can  be  attained.  If  too  many  coils  are  put  into 
a  given  fixed  length  of  spring  the  theoretical  deflection  required  of  each 
coil  will  be  within  safe  limits,  but  there  will  not  be  sufficient  free  space 
between  the  coils  to  permit  even  the  small  necessary  movement  or 
compression,  and  when  the  pitch  is  thus  too  flat,  the  spring  will  have 
insufficient  reactive  power  or  force,  because  of  the  inadequate  strain 
and  fibre  stress  put  upon  the  steel.  The  spring  must  thus  have  sufficient 


SAFETY  YALYE  SPRINGS 


7 


force  to  make  the  valve  open  and  close  promptly  and  positively  and 
keep  the  seat  tight,  not  only  to  give  prompt  relief  but  to  prevent  the 
constant  simmering  and  leaking  which  cuts  and  destroys  the  seats  and 
permits  the  deposit  of  boiler-scale  upon  any  exposed  threads.  The 
requirements  of  positive  control  and  extreme  lift  are  thus  to  a  large 
degree  contradictory. 

The  character  in  action  and  performance  required  of  the  spring 
in  a  pop  safety  valve  is  different  from  that  expected  of  car  springs  or 
similar  buffers.  In  carrying  the  load  of  a  car  or  wagon,  any  unevenness 
of  the  road  causes  a  jolting  or  bouncing,  and  the  momentum  of  the  mov¬ 
ing  car  adds  temporarily  to  the  effect  of  its  dead  weight  to  increase  the 
violent  action  of  the  spring.  Under  severe  conditions  such  springs  are 
often  compressed  to  their  limit,  until  the  coils  are  in  solid  contact,  and 
a  severe  bump  or  jar  is  felt  in  the  car  itself;  but  the  reaction  of  the  spring, 
when  the  unevenness  is  past,  sets  the  car  back  to  its  proper  position,  and 
on  the  rebound  it  may  be  that  the  car  has  risen  above  its  normal  place 
so  that  the  spring  is  drawn  out  in  tension;  after  a  while  these  waves 
of  oscillation  subside,  and  the  car  rides  normally.  For  such  service, 
car  springs  are  generally  designed  so  that  when  the  car  is  loaded  in 
ordinary  mapner  the  spring  will  be  about  one-half  compressed,  or  one- 
half  of  its  free  movement  will  carry  or  balance  the  contemplated  load. 
The  reason  is  that  this  gives  some  leeway  for  additional  load  in  the 
car  without  compressing  the  spring  too  much,  and  also  permits  the  great¬ 
est  movement  above  and  below  the  normal  medium  position.  It  is 
believed  that  the  conditions  of  longest  life  and  least  severe  fracturing 
strain  upon  the  car  spring  are  met  when  the  calculated  ordinary  load 
will  thus  compress  the  spring  one-half  of  its  total  free  movement. 

For  pop  valve  service,  on  the  other  hand,  the  conditions  are  different; 
the  exact  pressure  load  is  determined  from  the  exposed  area  of  the 
disc  and  the  steam  pressure,  and  when  the  valve  opens  there  is  an 
added  load  governed  by  the  additional  area  of  the  disc  and  the  steam 
pressure,  which  rapidly  decreases  as  the  steam  directly  below  the  valve 
escapes  and  the  boiler  is  relieved.  Under  no  conceivable  conditions  of 
actual  service  can  sufficient  steam  pressure  be  brought  upon  the  valve 
disc  to  compress  the  spring  so  that  the  coils  will  be  solid,  metal  to 
metal,  if  it  has  been  reasonably  designed  for  its  original  fixed  load; 
and  the  additional  spring  compression  to  permit  the  valve  opening  in 
order  to  relieve  the  boiler  is  comparatively  little,  possibly  0.08  inch, 
more  commonly  and  preferably  less,  and  never  under  any  conditions 
as  much  as  0.12  inch  or  say  ^  inch  as  an  extreme  amount.  If  after  the 


8 


SAFETY  VALVE  SPRINGS 


fixed  load  pressure  is  reached,  the  spring  has  still  15-32  inch  of  unused 
possible  compression,  of  which  less  than  3-32  inch  will  be  required  to 
accommodate  the  desired  lift  of  the  valve,  there  will  be  still  3-8  inch 
more  before  the  spring  would  go  solid  at  which  point  all  further  com¬ 
pression  would  be  impossible ;  therefore  the  valve  spring  can  be  properly 
designed  to  carry  its  set  load  at  much  more  than  half  of  its  total  free 
compression,  and  nearer  to  its  solid  condition  than  would  be  wise  with 
a  car  spring  where  the  amount  of  amplitude  of  total  motion  is  not 
limited. 

If  springs  are  properly  proportioned,  the  point  of  greatest  resilience, 
elasticity  and  reaction,  securing  sharp  action  in  the  valve,  and  permitting 
the  most  sensitive  and  accurate  adjustment  of  opening  pressure,  is  in 
the  last  one-third  of  the  total  possible  free  compression,  and  this  is  the 
part  of  the  spring  action  which  should  be  utilized  for  safety  valve  service. 
I  believe  it  proper  to  proportion  the  spring  so  that  the  set  load  is  carried 
at  about  two-thirds  or  three-fourths  of  its  total  free  compression, 
making  the  length  and  dimensions  of  the  spring  such  that  the  remaining 
unused  compression  of  the  spring  will  be  ample  for  the  lift  of  the  disc, 
and  a  safe  margin  beyond. 

While  it  may  be  said  that  the  springs  will  never  be  subjected  to 
the  extreme  compression  required  to  force  them  solid  in  service,  yet 
the  working  compression  is  a  large  proportion  of  the  total  free  move¬ 
ment  and  the  spring  might  be  dangerously  near  the  point  of  setting  or 
fracture  unless  properly  proportioned  and  tempered  to  take  the  solid 
test.  In  making  boiler  tests  the  head  bolt  may  be  set  down  until  the 
spring  is  solid,  to  close  the  valve;  and  if  the  valve  is  fitted  with  a  lever, 
the  spring  may  be  at  any  time  lifted  or  compressed  an  indefinite  amount 
by  that  means,  even  to  solid.  I  would  not  consider  it  proper  to  use  in 
a  pop  safety  valve  any  spring  that  would  not  safely  take  the  solid 
test,  capable  of  being  compressed  until  the  coils  are  metal  to  metal  any 
number  of  times  consecutively,  without  showing  any  permanent  set  or 
strain.  Otherwise  there  will  be  nothing  to  prevent  the  spring  from 
being  screwed  down,  even  through  ignorance,  until  the  danger  point 
may  be  once  passed,  and  the  spring  then  takes  a  permanent  set,  after 
which  it  becomes  entirely  ineffective  as  a  valve  spring  and  a  source  of 
danger  if  its  use  is  ignorantly  continued. 

One  prominent  manufacturer  of  safety  valves  requires  all  springs 
to  be  designed  to  take  this  solid  test  indefinitely.  After  the  springs  are 
made  and  tempered,  they  are  closed  solid  in  a  press  at  least  three  times; 
and  again  before  they  are  put  into  valves  for  service,  at  the  time  when 


SAFETY  VALVE  SPRINGS 


9 


the  ends  are  dressed  and  fitted,  they  are  tested  three  times  solid  and  the 
compression  at  the  proper  load  for  which  they  are  designed  is  noted. 
If  any  spring  shows  a  set  or  shortening,  even  temporarily,  of  as  much 
as  1-16  inch  in  any  of  these  tests,  or  if  there  is  much  variation  in  the 
compression  at  normal  load,  it  is  condemned  and  rejected.  Out  of 
the  great  number  of  valve  springs  made  and  tested  every  year  under 
these  conditions  less  than  one-third  of  one  per  cent  are  rejected  on 
this  account,  which  shows  that  the  requirements  are  commercially 
practicable  and  that  the  method  of  calculation  and  design  is  sound  and 
conservative.  Breakage  of  these  springs  in  service  is  practically 
unknown.  Such  results  demonstrate  what  modern  methods  with  care 
and  accuracy  can  accomplish,  in  producing  uniformity  of  performance 
and  reliability,  and  would  be  remarkable  in  any  manufacturing  opera¬ 
tions  where  the  number  of  pieces  is  so  considerable,  without  taking 
into  account  the  peculiarities  of  springs  and  steel. 

If  after  the  winding,  tempering  or  testing,  the  finished  spring  is  a 
little  shorter  than  the  standard  length,  there  is  a  temptation  to  reheat 
it  and  pull  it  out  a  little  to  make  it  the  full  length,  but  such  treatment 
is  undesirable  even  if  it  be  so  skillfully  done  that  the  steel  is  not  burned 
or  overtempered.  Not  all  springs  so  reheated  would  be  inferior,  but 
perfect  security  lies  in  accepting  the  small  losses  that  may  occur  rather 
than  assume  any  risk. 

Springs  of  comparatively  poor  design,  if  well  made  of  proper  steel, 
heated  uniformly  to  the  temperature  for  working  and  tempered  skill¬ 
fully,  are  to  be  preferred  to  springs  of  better  dimensions  but  improperly 
made.  In  most  small  shops  where  springs  are  wound  by  hand,  the 
long  bar  of  steel  must  be  heated  in  comparatively  short  sections  in  a 
small  furnace  or  forge  fire,  winding  about  a  foot  of  the  steel  at  each 
operation,  drawing  the  bar  by  hand-tongs  around  a  cold  mandrel  and 
then  sticking  the  remainder  of  the  straight  bar  again  into  the  fire  until 
it  is  soft  enough  to  wind  another  coil  or  two.  Each  foot  of  the  steel 
has  been  heated  to  a  different  temperature,  and  where  the  heated 
sections  have  overlapped,  the  steel  is  likely  to  be  burned.  The  same 
difficulty  arises  in  tempering,  where  it  is  necessary  to  move  a  large 
spring  rapidly  around  in  the  fire  to  try  to  get  all  parts  of  it  heated  up 
at  the  same  time;  one  end  sticks  out  into  the  cold  air  while  the  other 
end  is  in  the  hottest  part  of  the  fire  in  the  blast.  No  two  springs  made 
in  this  way  can  be  alike,  and  different  coils  of  the  same  spring  will  differ 
considerably  in  load-strength,  temper,  elasticity,  resilience  and  breaking 
strain,  for  generally  about  one  coil  is  all  that  can  be  wound  at  each 


10 


SAFETY  VALVE  SPRINGS 


heating.  The  bars  measure  from  five  to  twelve  feet  long  for  making 
springs  for  valves  of  the  larger  sizes,  and  for  locomotive  valves  the 
lengths  would  be  about  four  to  six  feet.  The  whole  bar  of  steel  should 
be  evenly  heated  at  one  time,  without  being  exposed  to  any  direct 
de-carbonizing  flame  or  forge  blast,  then  wound  accurately  and  quickly 
at  one  operation,  and  plunged  in  the  tempering  bath  at  exactly  the 
proper  moment. 

These  springs  should  be  wound  of  a  special  grade  of  steel,  kept  up 
to  standard  specifications,  in  bars  of  the  various  sizes  and  shapes 
required  for  different  loads  and  pressures ;  and  although  they  are  wound 
in  the  same  general  manner  and  have  much  the  same  outward  appear¬ 
ance  as  ordinary  coiled  helical  springs  that  are  used  for  car  springs  and 
other  commercial  purposes,  in  reality  they  are  very  different  in  treat¬ 
ment  and  character,  and  proper  results  can  never  be  obtained  if  springs 
of  ordinary  steel  are  substituted.  No  railroad  should  attempt  to  use 
car  springs  of  similar  shape  that  may  be  on  hand,  or  to  buy  springs  for 
safety  valves  by  specifying  simply  the  measured  dimensions. 

In  measuring  springs,  it  is  the  custom  and  better  practice  to  state 
first  the  inside  or  mandrel  diameter,  then  the  free  length  of  the  finished 
spring  and  last  the  diameter  and  form  of  the  steel  rod  used,  the  measure¬ 
ment  of  the  cross-section  being  that  of  the  straight  bar  before  winding. 
For  long  springs  it  is  sometimes  necessary  to  use  a  taper  mandrel  to 
facilitate  the  drawing  off  of  the  spring,  although  generally  the  slight 
natural  expansion  of  the  spring  after  winding  will  sufficiently  release  it; 
where  the  mandrel  is  tapered,  the  mean  diameter,  approximately  half¬ 
way  between  the  two  ends,  is  the  dimension  to  be  specified.  The  over¬ 
all  outside  diameter  will  generally  be  slightly  more,  and  the  outer  face 
of  the  coiled  square  steel  will  be  less,  than  the  commercial  dimensions 
would  indicate. 

In  any  design  and  calculation  of  springs  the  factor  of  proper  or 
permissible  fibre  stress  must  enter;  and  it  is  obvious  that  this  amount 
of  fibre  stress  can  be  stated  or  calculated  to  be  any  quantity  which 
may  be  desired  or  may  seem  comfortable  or  convenient  in  the  judgment 
of  the  engineer,  by  properly  proportioning  the  various  assumed  factors 
and  constants,  which  must  be  determined  more  or  less  arbitrarily  in 
operating  under  any  formula  whatever.  Whether  we  reason  from 
Hooke’s  Law,  or  from  the  general  statement  that  every  action  has  an 
equal  and  opposite  reaction,  or  appeal  directly  to  the  doctrine  of  the 
conservation  of  energy,  it  is  plain  that  we  can  get  no  more  work  out  of 
a  spring  in  reaction  than  is  put  into  it,  and  the  more  force  or  power  in 


SAFETY  VALVE  SPRINGS 


11 


compression  that  is  put  into  a  spring  of  given  dimensions,  the  greater 
the  amount  of  work  which  it  will  return  and  the  sharper  and  more 
positive  will  be  its  action  in  controlling  the  valve.  The  way  to  get  proper 
work  out  of  a  spring  is  to  put  force  into  it  in  effective  stress. 

In  a  paper  by  a  member  of  the  Society,  Mr.  H.  V.  Wille,  of  the 
Baldwin  Locomotive  Works,  printed  in  the  January  number  of  the 
Journal  of  the  American  Society  of  Mechanical  Engineers,  he  discusses 
difficulties  that  have  been  met  with  in  designing  stay  bolts  for  locomotive 
fire  boxes,  where  by  reason  of  the  expansion  the  bolts  are  stressed  above 
the  elastic  limit,  in  flexure  more  than  in  tension.  He  argues  that  the 
obvious  remedy  does  not  lie  in  the  use  of  a  slow-breaking  material  but 
in  the  employment  of  steel  of  sufficiently  high  elastic  limit  to  meet 
the  conditions  of  service,  and  thus  also  to  reduce  the  necessary  diameter 
of  the  bolt  and  proportionately  reduce  the  fibre  stress  in  flexure.  He 
states  that  stay  bolts  have  recently  been  made  of  the  same  grade  of 
steel  as  that  used  in  the  manufacture  of  springs,  oil-tempered,  to  safely 
stand  a  fibre  stress  of  100,000  pounds  per  square  inch,  its  high  elastic 
limit  making  it  possible  to  reduce  the  diameter  so  that  the  fibre  stress 
in  flexure  is  less  than  one-half  that  in  the  ordinary  type  of  bolt  and  the 
material  is  capable  of  being  thus  stressed  to  a  high  degree.  This  same 
reasoning  applies  aptly  to  the  problem  of  making  suitable  springs  for 
safety  valves,  and  practical  experience  has  demonstrated  that  every 
argument  and  consideration  is  in  favor  of  putting  the  steel  under  the 
highest  practicable  fibre  stress.  Safety  here  lies  not  in  allowing  a 
factor  too  large,  but  in  using  material  of  known  and  uniform  quality 
and  greater  strength. 

Low  fibre  stress  is  not  a  measure  of  safety  but  of  ineffectiveness. 
To  properly  develop  the  resilience  of  the  most  trustworthy  and  suitable 
steel  available  today  requires  a  stress  that  would  be  inadmissible  with 
material  of  inferior  temper  or  uncertain  quality  and  subject  to  ordinary 
commercial  defects.  Experience  shows  that  springs  may  best  be 
stressed  at  from  60,000  to  75,000  pounds  per  square  inch  at  the  fixed 
load,  which  should  compress  the  spring  about  70  per  cent  of  its  total 
possible  free  movement,  the  remaining  movement  should  be  three  or 
four  times  the  further  lift  of  the  valve  in  opening.  This  is  low  enough 
for  good  steel  and  gives  safe  working  limits  and  conditions,  and  under 
the  same  formula  the  stress  when  the  spring  is  solid  will  not  exceed 
100,000  pounds  per  square  inch. 

The  limit  of  elasticity,  beyond  which  a  permanent  set  occurs,  is 
different  for  torsional  strains  than  for  elongation  and  is  independent  of 


12 


SAFETY  VALVE  SPRINGS 


the  tensile  strength,  and  for  steel  of  the  proper  characteristics  for 
spring-making,  this  torsional  elasticity  is  relatively  high.  Car-springs 
have  sometimes  been  calculated  upon  a  fibre  stress  as  low  as  30,000 
pounds  per  square  inch  at  normal  load,  and  this  may  have  seemed 
reasonable  for  common  grades  of  steel  under  circumstances  where  the 
springs  might  be  frequently  subjected  to  the  extreme  compression  or 
extension.  But  the  material  to  be  used  today  does  not  properly 
develop  the  power  in  reaction  at  any  such  small  percentage  of  its  total 
strength,  and  the  fibre  stress  now  recommended  in  the  hand-books 
and  in  the  modern  text-books  on  applied  mechanics  is  generally  80,000 
pounds.  This  is  not  a  question  of  keeping  within  limits  of  safety, 
but  of  stressing  the  steel  to  its  point  of  proper  efficiency.  For  example, 
it  would  be  absurd  to  expect  a  spring  suitable  for  use  in  a  valve  at  200 
pounds  pressure  to  show  proper  performance  and  lift  or  permit  satis¬ 
factory  opening  and  closing  of  the  valve  at  only  50  pounds  pressure; 
its  reaction  and  resilience  could  not  be  at  all  reasonably  developed. 

If  a  spring  is  designed  upon  a  formula  which  may  indicate  a  fibre 
stress  of  70,000  pounds  per  square  inch  at  normal  load,  and  as  much  as- 
100,000  pounds  per  square  inch  when  compressed  solid,  yet  is  made 
of  such  steel  that  it  can  remain  assembled  in  the  valve  indefinitely 
under  pressure  without  perceptible  set,  and  can  be  compressed  solid 
an  indefinite  number  of  times  without  injury,  it  is  evident  that  it  is 
more  properly  proportioned  and  is  used  at  a  better  and  safer  percentage 
of  its  elastic  limit  than  springs  made  of  less  virile  steel,  that  although 
calculated  by  some  formula  which  indicates  only  30,000  pounds  per 
square  inch  fibre  stress  at  normal  load,  will  suffer  a  gradual  set  or 
deterioration  in  service  or  will  become  permanently  set  if  tested  solid. 
Of  course  it  is  obvious  that  certain  grades  of  steel,  whether  on  account 
of  cheapness  in  methods  of  manufacture,  lack  of  uniformity  in  character, 
or  poor  judgment  in  adaptation  to  the  special  purposes  in  hand,  may 
show  a  set  at  a  comparatively  early  point  beyond  the  normal  load,  no 
matter  at  what  figure  it  may  be  fixed;  and  therefore  a  fixed  limit  of 
fibre  stress  has  no  meaning  unless  the  characteristics  of  the  steel  be 
also  specified  or  known.  Springs  wound  of  bronze  are  notoriously 
inefficient  and  unenduring,  and  their  depreciation  and  permanent  set 
in  service  at  comparatively  low  fibre  stress  will  more  than  counter¬ 
balance  any  possible  advantage  of  slow  corrosion;  and  certain  grades 
of  steel  may  be  as  poorly  adapted  for  the  making  of  valve  springs. 
The  torsional  elasticity  and  power  depends  not  upon  the  tensile  strength 
so  much  as  upon  the  temper  and  resilience.  Therefore  some  of  the 


SAFETY  VALVE  SPRINGS 


13 


new  alloy  steels  have  proved  disappointing  for  this  service  and  the 
name  of  any  alloy  can  not  as  yet  be  used  either  as  a  fetich  or  a  selling 
phrase. 

Observation  of  many  thousand  springs  in  continuous  daily  service, 
under  severest  conditions  of  constant  use,  shows  that  springs  calculated 
upon  a  very  high  fibre  stress  are  entirely  reliable  for  indefinite  periods 
of  service  and  do  not  develop  any  measurable  percentage  of  faults  or 
fractures;  the  failure  of  valves  under  operating  conditions  attributable 
to  such  springs  is  practically  unknown  and  is  less  than  the  percentage 
due  to  the  rare  defects  of  the  other  structural  parts. 

The  theory  of  the  action  of  helical  springs  was  developed  between 
1814  and  1848  by  Binet  and  by  Professor  Thomson,  who  argued  mathe¬ 
matically  that  the  force  opposing  the  elongation  or  contraction  of  the 
spring  is  the  elasticity  of  torsion,  which  is  independent  of  the  elasticity 
of  compression  or  extension  in  the  rod;  but  all  mathematical  discussions 
of  the  nature  of  the  action  of  helical  springs  have  been  simplified  or 
made  possible  by  eliminating  or  disregarding  all  consideration  of 
certain  practical  conditions  met  with  in  practice.  Green  showed  in 
1830  that  the  general  investigation  of  the  relations  between  the  stresses 
and  deformations  of  any  solid  body  depends  in  its  simplest  form  upon 
the  solution  of  a  quadratic  equation  having  six  unknown  quantities 
and  twenty-one  terms  whose  coefficients  are  essential,  therefore  we 
may  pardon  all  the  later  investigators  who  have  refused  to  include  the 
further  complications  that  consideration  of  the  means  of  applying  the 
pressure  upon  the  ends  of  the  spring  and  the  flexural  stresses  would 
introduce,  and  have  made  a  short  cut  by  imagining  the  spring  unwound 
and  resolved  back  into  the  original  straight  bar  of  uniform  section, 
in  order  to  obtain  any  workable  formula  whatever;  and  we  can  under¬ 
stand  why  more  do  not  rush  in  with  easy  experiments  and  confident 
amendments. 

The  very  complete  mathematical  discussion  of  this  whole  subject 
by  St.  Venant,  the  French  physicist,  in  1855,  with  graphic  illustrations 
of  his  conclusions,  includes  the  consideration  of  round,  square,  triangular 
and  many  peculiar  sections.  He  demonstrates  what  may  seem  at  first 
rather  startling,  that  the  points  of  greatest  distortion  at  the  surface 
are  nearest  to  the  axis  of  the  twisted  section,  that  the  least  distortion 
occurs  at  the  points  farthest  from  the  center,  and  that  the  stress  and 
strain  tend  toward  zero  at  projecting  angles,  either  acute  or  obtuse. 
Thomson  and  Tait  followed  this  with  elaborate  mathematical  discussion 
and  practical  experiments.  The  more  important  flexural  distortions, 


14 


SAFETY  VALVE  SPRINGS 


in  bending  the  steel  and  winding  the  spring,  as  well  as  the  torsional 
strains,  are  along  the  outer  and  inner  faces  of  the  square  section  in  each 
coil  and  to  a  less  degree  along  the  lateral  faces,  at  both  the  short  di¬ 
ameters  of  the  section  and  not  through  the  corners;  as  may  be  very 
roughly  illustrated  by  winding  a  square  rod  of  soft  rubber  into  helical 
form. 

But  the  actual  spring  is  not  even  approximately  a  straight  bar 
held  firmly  at  one  end  and  twisted  uniformly;  and  the  pressure  is  not 
applied  at  the  extreme  end  of  the  bar  but  upon  a  flat  plate  or  thimble 
at  either  end,  fitted  when  the  spring  is  free  and  not  when  more  or  less 
distorted  under  load,  but  intended  to  transmit  the  pressure  as  nearly 
in  a  straight  line  along  the  axis  of  the  mandrel  as  conditions  will  permit. 
The  violent  distortion  of  the  cross-section  of  the  square  steel  rod,  which 
by  no  stretch  of  imagination  can  be  considered  as  still  rectangular, 
sets  up  internal  stresses  in  the  tempered  steel,  which  must  be  recognized 
even  if  they  cannot  be  conveniently  measured.  The  strain  of  deflection 
of  each  coil,  of  even  moderate  pitch,  amounts  to  a  serious  item,  but  is 
expressly  brushed  aside  and  ignored  in  deriving  the  ordinary  working 
formulae. 

Springs  should  be  very  gradually  and  uniformly  heated  without 
decarbonizing  the  surface  skin  by  exposure  to  any  flame,  and  the 
excessive  hardness  or  temper  of  the  surface  afterward  drawn  by 
reheating  to  a  relatively  low  temperature.  This  reheating  seems  to 
give  a  toughness  to  withstand  greater  tensile  strain  and  is  especially 
important  in  proper  consideration  of  the  generally  neglected  factor  of 
lateral  or  flexural  strain  which  becomes  so  large  when  any  set  or  fracture 
of  the  spring  occurs.  I  am  persuaded  that  much  of  the  virtue  and 
special  character  of  the  spring  lies  in  the  molecular  tension  and  condi¬ 
tion  of  the  surface  skin  after  tempering. 

Although  the  chief  element  in  the  power  of  a  spring  is  its  torsional 
strain  developed  in  the  compression,  and  the  flexural  stress  may  be 
comparatively  a  small  part  of  the  useful  power  or  resilience,  yet  the 
destructive  set  experienced  from  over-compression  in  springs  of  few 
coils  with  steep  pitch,  can  be  shown,  I  believe,  to  be  more  commonly 
due  to  exceeding  the  elastic  limit  in  the  bending  strains  than  to  any 
severe  strain  in  torsion;  and  this  flexural  stress  increases  as  steel  of 
larger  cross-section  is  employed;  but  no  one  has  yet  been  willing  to 
risk  a  statement  of  the  influence  or  effect  of  variation  of  pitch,  as  a 
definite  factor  in  any  generally  accepted  formula. 

In  springs  made  of  square  steel  it  may  very  well  be  that  the  re- 


SAFETY  VALVE  SPRINGS 


15 


heating  to  draw  the  surface  temper  affects  the  corners  most  and  the 
lateral  faces  only  slightly  less,  and  that  the  toughened  metal  in  the 
corners  backs  up  or  strengthens  the  harder  interior  portions ;  the  internal 
strains  would  be  relieved,  while  an  approximately  cylindrical  interior 
would  retain  a  high  temper  to  resist  the  truly  torsional  stress  of  com¬ 
pression.  The  slightly  variable  temper  in  the  corners  and  lateral  faces 
of  the  square  steel  may  be  the  better  disposed  and  adapted  to  accom¬ 
modate  the  several  varying  strains  in  a  helical  spring;  for  the  sym¬ 
metrical  form  of  the  round  steel,  even  when  the  temper  is  slightly 
drawn  after  hardening,  permits  little  relative  adjustment  of  the  several 
lateral  and  torsional  strains,  and  the  round  section  can  be  considered 
as  subject  to  considerable  unrelieved  internal  strain  in  flexure. 

The  spring  must  have  sufficient  compression  to  afford  the  amount 
of  valve-opening  fixed  upon  as  reasonable  and  practicable,  yet  be  kept 
within  the  least  amount  of  movement  that  will  satisfy  these  demands, 
for  every  spring  has  considerable  eccentricity,  depending  upon  the 
pitch  and  proportions  of  the  coils,  and  under  the  increasing  compression 
or  extension  as  the  valve  opens  or  closes,  the  ends  have  a  movement 
which  may  be  likened  in  some  degree  to  the  actions  of  the  free  end  of  a 
fire-hose  under  pressure.  The  side-thrust  due  to  this  twisting  and 
untwisting  eccentricity  is  transmitted  to  the  valve  disc  and  guide  wings, 
and  increases  rapidly  with  each  fraction  of  increased  lift  or  opening 
of  the  valve. 

Large  movement  of  the  spring  in  compression  is  undesirable;  it  is 
but  a  necessary  means  to  an  end,  an  evil  to  be  kept  within  minimum 
limits.  The  chief  object  of  a  safety  valve  is  not  to  lift  the  disc  but 
to  release  steam;  not  to  perform  internal  work  in  the  valve  but  to 
relieve  the  boiler.  It  would  be  an  advantage  if  satisfactory  discharge 
through  the  valve  could  be  attained  with  even  less  spring  compression 
than  at  present.  Large  lift  of  the  disc  is  not  merely  a  measure  of 
capacity  but  of  inefficiency;  for  the  valve  which  releases  the  steam 
with  the  least  proportional  lift  or  spring  compression  is  to  that  degree 
the  more  efficient  for  its  purpose  and  at  the  same  time  more  safe  and 
reliable. 

The  specifications  which  require  valve  seats  to  be  made  of  non- 
corrosive  metal  and  the  rules  which  compel  every  valve  to  be  tried 
and  lifted  by  the  lever  every  day,  aim  to  overcome  the  ever-present 
danger  that  the  valve  may  stick  upon  its  seat  and  fail  to  open  at  the 
critical  moment;  but  the  greatest  cause  of  the  sticking  of  the  valve, 
when  it  does  occur,  is  not  corrosion  of  the  seat  face  but  the  binding 


16 


SAFETY  YALYE  SPRINGS 


friction  of  the  disc-guides  against  the  side  of  the  well  or  throat  of  the 
valve.  This  cocking  or  binding  effect  can  be  decreased  by  any  modifica¬ 
tion  of  design  which  will  reduce  the  diameter  of  the  cylindrical  guide, 
or  which  will  eliminate  the  friction  of  the  lower  ends  of  the  guides  and 
bring  the  guiding  surface  close  to  the  plane  of  the  seat;  both  of  these 
modifications  reduce  the  moment  of  the  friction  or  cocking  stress. 
Any  device  which  reduces  the  lift  of  the  disc  and  the  spring  movement 
to  the  least  possible  amount  will  also  reduce  the  eccentric  spring  action 
and  its  effect,  and  of  course  any  valve  design  which  requires  or  con¬ 
templates  an  unnecessarily  large  lift  or  compression  disadvantageously 
magnifies  this  effect.  It  is  perhaps  important  to  point  out  that  as  the 
primitive  and  still  common  form  of  safety  valve  has  a  seat  opening 
beveled  at  an  angle  of  45  degrees  with  the  vertical  spindle,  the  effective 
passage  through  the  seat  is  measured  by  the  sine  of  45  degrees  and  is 
approximately  only  0.7  of  the  actual  vertical  lift  or  compression  of 
the  spring  when  the  valve  opens,  so  that  the  spring  must  necessarily 
compress  about  one  and  one-half  times  the  effective  lift,  and  even  this 
does  not  always  afford  a  free  passage  for  the  steam  to  the  air  where 
there  is  vertical  overlap  of  the  regulating  ring  against  the  lip  of  the 
disc  in  order  to  increase  the  lift  against  the  greater  pressure  of  the 
shortening  spring. 

In  the  well-known  annular  type  of  valve,  the  area  of  the  disc 
between  the  seats  open  to  the  constant  pressure  of  the  steam  is  ap¬ 
proximately  only  four-fifths  of  the  total  initial  area  of  the  disc  under 
load  in  the  bevel-seated  form  of  valve  having  the  same  diameter  and 
seat  circumference;  therefore  the  use  of  the  familiar  annular,  flat-seated 
valve  is  the  logical  way  to  reduce  to  a  minimum  all  the  difficulties  of 
spring  making,  especially  where  the  space  available  for  the  spring  is 
absolutely  limited  by  the  over-all  dimensions  permitted  by  locomotive 
builders  and  boiler  makers;  for  the  spring  need  thus  be  of  dimensions 
and  strength  to  carry  only  four-fifths  of  the  load  necessary  in  the  lip- 
type  of  valve;  the  vertical  lift  and  spring  compression  require  to  be 
only  0.7  as  much,  or  for  the  same  lift  will  give  one  and  one-half  times 
as  much  discharge  area;  and  no  preliminary  lift  is  required  to  relieve 
the  overlap  of  an  adjusting  ring,  for  the  work  of  giving  to  the  disc  its 
sudden  pop  lift  is  performed  by  an  auxiliary  steam  discharge  by-passed 
through  the  central  passages.  This  by-passed  or  auxiliary  discharge 
adds  its  volume  to  the  main  discharge  capacity  and  leaves  an  absolutely 
unrestricted  and  unthrottled  free  escape  for  the  main  flow  of  the  released 
steam  directly  to  the  open  air,  without  any  tortuous  expansion  chamber 


SAFETY  VALVE  SPRINGS 


17 


or  deflecting  ring;  the  outlet  is  across  a  flat  seat  which  not  only  utilizes 
the  full  vertical  lift  but  gives  a  discharge  opening  of  cylindrical  form 
with  efficient  rounded  edges.  This  form  of  disc  has  the  further  ad¬ 
vantage  that  it  is  impossible  for  it  to  jam  or  stick  and  it  is  easily  refaced 
by  rubbing  on  a  face-plate  instead  of  grinding  to  a  bevel;  and  as  the 
disc  can  be  made  entirely  without  the  ever-cocking  and  sticking  guides, 
the  efficiency  of  the  spring  can  be  thus  aided  and  utilized  to  the  greatest 
possible  degree.  My  emphasis  upon  this  point  of  spring  limitation  and 
the  helpful  effect  of  suitable  valve-seat  design  is  because  attention  has 
not  been  generally  called  to  its  importance  and  experience  has  shown 
the  serious  difficulties  which  may  be  thus  minimized  if  not  entirely 
avoided. 

It  is  not  within  my  purpose  to  recommend  any  definite  formula 
for  the  calculation  of  helical  springs.  My  investigations  have  led  me 
to  believe  that  there  are  not  in  this  country  today  many  men  who  have 
experienced  the  fortuitous  concurrence  of  time,  inclination,  business 
necessity  and  proper  manufacturing  and  testing  facilities  to  lead  them 
to  develop  a  practical  working  formula  very  far  beyond  the  very  un¬ 
satisfactory  rules  laid  down  in  the  hand-books.  Experiments  have 
been  carried  out  upon  springs  wound  of  comparatively  small  wire, 
but  every  one  who  has  had  occasion  to  use  the  conventional  formulae 
must  have  realized  that  no  matter  how  well  they  cover  a  few  types  of 
car-springs  within  a  limited  range,  they  lead  us  far  astray  in  this  special 
branch  of  the  problem;  especially  in  cases  where  the  limitations  upon 
the  free  length  of  the  spring  will  not  permit  the  use  of  either  round  or 
square  bars  and  some  special  flat  or  rectangular  section  must  be  used 
to  secure  the  required  area  of  steel  and  still  leave  room  for  movement 
between  the  coils. 

One  engineer  to  whom  I  wrote  replied :  “  I  would  say  in  reference  to 
the  questions  raised  by  you,  that  I  do  not  consider  it  good  practice  to 
use  a  stress  of  over  95,000  or  100,000  pounds  for  valve  springs,  although 
we  have  in  our  various  experiences  stressed  springs  as  high  as  145,000 
and  150,000  pounds,  but  we  do  not  under  any  circumstances  recom¬ 
mend  this  or  any  approximate  stresses  for  the  extreme  service  to  which 
valves  are  subjected.  Regarding  this  question  of  stress,  I  would 
further  say  that  a  long  time  ago  we  demonstrated  that  while  various 
published  formulae  are  correct  within  certain  limits  for  springs  made  of 
round  steel,  they  were  far  from  being  correct  for  springs  made  of  squares 
and  special  sections.  We  devoted  a  great  deal  of  time  to  both  theory 
and  experiment  to  demonstrate  the  formula  which  we  now  use,  and 


18 


SAFETY  VALVE  SPRINGS 


which  we  have  proved  by  exhaustive  experiments  to  be  correct. 
We  know  that  by  using  numerous  published  formulae  that  some  springs 
will  show  very  excessive  stress;  however,  these  excessive  stresses  are 
not  actual,  but  apparent,  due  to  errors  in  the  formula.  The  formula 
which  we  use  being  the  result  of  years  of  experience,  experiment  and 
calculation,  you  can  readily  understand  is  not  published,  and  further¬ 
more,  would  be  useless  without  the  accumulation  of  data  and  special 
information  which  we  have.” 

My  aim  has  been  rather  to  state  conclusions  derived  from  experience 
and  observation  and  from  the  counsel  and  advice  of  those  oldest  and 
wisest  in  this  work  as  well  as  from  an  inheritance  of  the  results  of  thirty 
years  of  special  study  and  experiment  in  this  limited  field.  I  have 
reviewed  the  several  practical  considerations  that  must  be  taken  into 
account,  so  that  in  the  discussion  of  the  general  subject  this  important 
element  of  the  spring  may  have  its  due  attention. 

Nearly  every  engineer  of  education  and  experience  would  be 
qualified  and  perhaps  ready  to  suggest  at  least  one  apparently  simple 
and  more  or  less  obvious  improvement  in  the  design  of  safety  valves  or 
springs,  but  nearly  every  such  possible  detail  will  be  found  to  be  already 
old.  Almost  every  conceivable  device  and  modification  has  been  the 
subject  of  a  patent,  and  most  of  these  have  been  thoroughly  tried, 
with  much  expense  and  enthusiasm,  before  being  condemned  and  dis¬ 
carded.  The  various  subterfuges  of  double  or  concentric  springs,  one 
more  flexible  than  the  other,  of  spiral  springs  with  coils  of  increasing 
diameter,  whose  first  movement  in  compression  is  rapid  until  the  smaller 
and  stiffer  coils  are  brought  into  action,  springs  suspended  in  all  sorts 
of  universal  bearings,  and  every  method  of  end  bearing  and  fitting, 
have  all  been  tried  and  abandoned  by  almost  every  maker  and  user  of 
safety  valves,  but  are  periodically  revived  or  rediscovered  by  some  new 
designer  or  foreman  and  brought  forward  for  consideration  again  and 
again.  Before  any  such  expedients  are  advocated,  a  study  of  the 
old  file  of  long-expired  patents  would  be  enlightening  and  interesting. 
Attempts  at  using  extension  springs,  in  place  of  springs  to  be  compressed 
as  the  valve  opens,  can  be  seen  in  patents  and  experimental  valves  of 
twenty-five  years  ago;  but  the  idea  has  been  generally  abandoned,  for 
not  only  is  it  difficult  to  devise  any  satisfactory  end-fastening  which  will 
take  firm  hold  of  the  steel  without  inducing  a  fracture  there,  but  such  a 
spring  could  be  easily  and  ignorantly  overstrained  and  set,  wdiile  a 
safely  proportioned  compression  spring  can  not  take  a  set  within  its 
possible  limit  of  movement,  even  to  solid;  and  the  side-thrust  due  to 


SAFETY  YALYE  SPRINGS 


19 


unevenly  disposed  coils  is  found  in  extension  springs  also,  wherever  the 
springs  have  not  the  suitable  length  to  permit  proper  proportioning  and 
balance. 

It  would  serve  no  useful  purpose  were  each  engineer  to  attempt 
to  specify  the  exact  dimensions  of  the  springs  to  be  used  in  safety  valves 
for  each  pressure  and  service,  for  the  limitations  of  special  cases  and  the 
several  conditions  to  be  taken  into  account,  with  their  relative  im¬ 
portance  and  weight  in  the  problem,  would  require  the  exercise  of 
judgment  to  be  derived  only  from  personal  experience,  and  any  general 
rule  would  be  burdened  with  a  long  catalogue  of  exceptions  more 
troublesome  than  helpful.  But  the  general  principles  rest  upon  a  solid 
foundation,  and  any  one  may  judge  whether  the  springs  in  his  safety 
valves  satisfy  certain  rational  requirements;  but  we  must  not  expect 
to  reconcile  mechanical  contradictions  nor  ask  for  valves  that  would 
demand  miracles  in  springs. 


